Flash EPROMs, for example, are used for the storage of program code in electronic units such as, for example, engine control units. It is also common in modern controllers to integrate flash memory elements to execute programs as quickly as possible. However, the integrated flash memory element is substantially more expensive than an external flash memory IC (integrated circuit) which, however, is slower in comparison with the former. For this reason, external (stand-alone) flash memory ICs are used for program sections which do not affect the performance of the overall system.
This creates the problem that different technologies are used to manufacture the different ICs for the computers and the memories which therefore run on different operating voltages. Even on the computer, depending on the technology, the operating voltages for the computer kernel and the operating voltage of the flash memory may be different.
Generally, the operating voltage of the flash memory is higher than that of the computer kernel. However, program execution is ensured only when the operating voltage for the memory in which the program to be executed is stored and the operating voltage for the computer kernel are within a specified tolerance zone.
Low voltage recognition of the operating voltages and associated error handling are necessary for a supply voltage dip of the engine control unit.
A method and a device for monitoring an electronic computing unit are described in German Patent Application No. DE 196 01 804. In the method described, in the event of an interruption in the supply voltage and following initialization of the computing unit, a reset signal is transmitted from a voltage monitoring device to the computing unit and, after the signal is received, the operating state of the computing unit is checked by comparing a content of a volatile memory cell with a fixedly predefined code. A decision regarding the further control sequence is made based on this check. After triggering a reset signal, the content of a volatile memory cell is compared with a predefined code. Each reset signal thus triggers a check during which a test is made on whether the content of the volatile memory is still completely present. This makes it possible to differentiate between interruptions in the voltage supply which cause errors in the computing unit and such interruptions in the voltage supply which do not cause errors in the computing unit.
Depending on the size of a dip in the supply voltage it may occur that the highest operating voltage is no longer sufficient for part of the flash memory, but the other operating voltages for the other parts of the flash memory and the computer kernel are still within the specified tolerance. Normal program execution is thus no longer possible under these circumstances because it is assumed that the entire flash memory must always be available.
A low voltage on the operating voltage is normally detected via a threshold value using hysteresis. However, a prerequisite for this is that the threshold value is higher than the memory's minimum operating voltage, but also lower than the lowest operating voltage caused by interruptions. Such an analysis is indeed no longer possible when the flash memory's operating voltage has a very narrow tolerance.